FIG. 1 contains a block diagram illustrating the elements of an exemplary pulse width modulation (PWM) amplifier 100 that includes a PWM controller 102, an output stage 128 and an optional output filter 130. The PWM amplifier 100 is shown as being used to drive a load 132, which can be a loudspeaker. The PWM Controller 102 is typically a monolithic integrated circuit (IC) device including an input/output (I/O) interface 104, a digital signal processor (DSP) core 106 for signal processing, and a PWM engine 110.
The I/O interface 104 receives a pulse code modulated (PCM) audio signal, typically at audio sample rates such as 44.1 kHz, 48 kHz, 96 kHz, or 192 kHz. It will typically support a variety of audio input formats such as S/PDIF, I2S, or HDA. The DSP core 106 receives audio samples from the I/O interface 104, performs signal processing effects such as crossovers, tone controls, or equalizers, and transfers the resulting digital audio signal 108 to the PWM engine 110.
The digital audio signal 108 provided to the PWM engine 110 will often be referred to hereafter as the audio data signal 108, which is the data signal input to the PWM engine 110. The PWM engine 110 will perform additional signal processing and the PCM-to-PWM transformation. The PWM engine 110 includes an interpolator block 112, a PWM correction block 120, a noise shaper 122, an optional clipper 124, and a PWM modulator 126.
In the exemplary design of FIG. 1, the audio data signal 108 can be, e.g., a 24 bit 2's complement pulse code modulated (PCM) audio data signal having an audio sampling frequency of 48 kHz. Further exemplary audio sampling frequencies include 44.1 kHz, 96 kHz and 192 kHz, but other sampling frequencies are also possible.
The audio data signal 108 (e.g., having an audio sampling frequency of 48 kHz) is then interpolated by an interpolator block 112 up to a predetermined pulse width modulation (PWM) rate. For example, the interpolator block 112 can upsample the audio data signal 108 (e.g., received from the DSP core 106) from a DSP sampling rate, e.g., 48 kHz, to a PWM switching rate, e.g., 384 kHz. As shown in FIG. 1, the interpolator block 112 can include, e.g., a front end interpolator 114, a back end interpolator 116 and a gain stage 118, but is not limited thereto. The front end interpolator 114 can interpolate up the sampling frequency by up to 4× and can include, e.g., a finite impulse response (FIR) filter with a relatively sharp cutoff. The backend interpolator 116 can interpolate up the sampling frequency by up to another 4× and can include, e.g., a simple inexpensive spline interpolator with a relatively sloppy cutoff. The interpolator gain stage 118 can, e.g., be adjusted to compensate for non-unity gain in the interpolator block 112, or used to adjust the gain of the audio data signal 108.
The output of the interpolator 112 is provided to a PWM correction block 120, which is also known as a non-linear correction block. The PWM correction block 120 applies a pre-correction (also know as pre-distortion) to the digital audio signal that approximately corrects for the non-linear artifacts created by the PCM-to-PWM conversion.
The pre-corrected (also known as pre-distorted) digital audio signal output by the PWM correction block 120 is then noise-shaped by the noise shaper 122. The noise shaper 122 can use predetermined stored noise shaper filter coefficient values and a quantizer to reduce the bit resolution, e.g., to the range of about 8-14 bits. For a specific example, the noise shaper 122 can quantize each 24-bit PCM digital audio sample to a 10-bit PCM digital audio sample and use noise shaping techniques to reduce the quantization noise within the audio band of interest (the audio band of interest can also be referred to as “in-band”), typically DC to 20 kHz or 40 kHz. The noise shaper 122 can be, e.g., a seventh order noise shaper, but is not limited thereto. A seventh order noise shaper can, e.g., use twenty-one separate 14-bit noise shaper filter coefficients to shape quantization noise, where each coefficient can have a value ranging 0 to 2{circumflex over (0)}14 (which may or may not be signed values, depending on implementation). The noise shaper 122 effectively moves the additional quantization noise introduced by the reduction of the signal bit resolution out of the audio band of interest so that the dynamic range of the bandwidth of interest is not limited to the bit resolution of the output data.
An optional clipping block 124 is shown between the noise shaper 122 and a PWM modulator block 126. The clipping block 124 can selectively clip signals output by the noise shaper 122 based on over-current detection (e.g., as detected by an over-current detector, not shown), in order to provide amplifier and load protection.
The PWM modulator block 126 performs a PCM-to-PWM conversion on the digital audio signal (e.g., a 10 bit signal) it receives and generates PWM output signals. The PWM output signals are used to drive the output stage 128, with can include, e.g., a pair of high voltage power FETs. The output of the high voltage power FETs can be optionally filtered by a filter 130, e.g., an LC filter, to remove a switching carrier and remove out-of-band noise. The filter 130 can be part of the amplifier, or external to the amplifier. The filtered (or non-filtered) output of the FET's is then applied to the load 132. This load 132 is typically a loudspeaker that converts the filtered output of the FET's to an audible signal. In a specific embodiment, the output stage is a 3-level PWM output stage, in which case the optional filter 130 is not needed, and thus, is typically not included. The output stage 128 is typically powered by a relatively high voltage (HV+), and thus, it is often the highest power consuming stage of the amplifier 100. Different output stages and filters than shown in FIG. 1 may be used.
As mentioned above, the noise shaper 122 performs digital filtering and quantizing using predetermined values for noise shaper filter coefficients and a quantizer. Normally, the noise shaper 122 uses its quantizer to perform quantization of the audio data signal 108 (after the audio data signal 108 has already been upsampled and pre-corrected) and shapes noise resulting from quantization of the audio data signal using the predetermined noise shaper filter coefficient values (also referred to as predetermined values for noise shaper filter coefficients, or predetermined values of noise shaper filter coefficients) in order to allow for an increase in dynamic range in a specific bandwidth of interest. This moves the added quantization noise power out of the audio band of interest, and the quantization noise power (which is out-of-band noise) is dissipated in the load.
Typically the noise shaper 122 reduces the in-band quantization noise of the digital PWM amplifier system. However, when the audio data signal 108 drops below the in-band noise floor of the noise shaper 122, the noise shaper 122 is the limiting factor of the performance as opposed to the quantization noise added by the noise shaper 122. A filter (e.g., filter 130) can reduce the out-of-band noise power, but in a filterless design, most of the out-of-band noise power is dissipated directly into the load 132. The audio data signal 108 may drop below the in-band noise floor of the noise shaper, e.g., when an idle channel condition occurs.